Neisseria gonorrhoeae, the causative agent of gonorrhoea, expresses one or more members of the opa multigene family during the course of disease. We have studied the expression mechanisms and the biological properties of the expressed outer-membrane proteins to more clearly understand the role of this protein family in the pathogenesis of gonorrheal disease. Phase variation (on/off expression changes) occurs through a translational frameshifting mechanism dependent on "non-homologous" recombination in a series of direct, tandem repeats within each opa gene. This mechanism has been used as a paradigm for a number of other neisserial genes which phase vary using similar repetitive stretches of DNA. We have determined a number of cellular functions, primarily DNA replication and repair enzymes, which significantly influence the rates at which opa genes (and others using similar mechanisms) phase vary. Attempts to mutate these genes in N. gonorrhoeae are underway with the primary goal of creating bacterial strains which can no longer regulate phase variation of these genes at normal levels. Expression of outer-membrane Opa proteins influence the adhesive properties of N. gonorrhoeae towards epithelial cells and PMNs. We have studied the nature of the adherence and have shown that certain Opa proteins bind a family of compounds termed glycosaminoglycans, found on the surface of numerous cell types and in the external mucosal environment. We have also shown that the expression of Opa proteins may protect Opa+ bacteria from the bactericidal components of human serum in the presence of human urine. This effect is similar to that seen with added heparin and studies are underway to further characterize the nature of the protective components in human urine.